Energy storage is one of the largest challenges of modern society. This applies to all amounts of stored energy, ranging from small devices (e.g., watches, cell phones or portable communication devices, and laptop computers) to large power-consuming entities, such as cities or factories. For large scale storage, a common method to harvest energy is to pump water into an elevated reservoir and use its released potential when flowing back to a deeper level. The main drawback of this technique is its relatively low storage energy capacity, which dates back to the first power plants built over a century ago.
Conventional batteries operate by converting chemical to electrical energy. In operation, the electronic current can be used to drive an external device. Unfortunately, such batteries are not easily scalable and suffer from degradation over time, with less and less energy being stored on continuous use.
Flow batteries emerged in the 1970s and offer scalable energy storage. These types of batteries are based on an electrolyte that is pumped through a system. Two separate flow routes meet at a point where electrons are transferred from one electrolyte to the other as a result of applied voltage (i.e., the electrical energy is transformed and stored as chemical energy). The two electrolytes (now charged positively and negatively, respectively) are stored in separate containers, or reservoirs. Having them flow back to a juncture point where electron transfer is possible, the chemically stored energy is harvested as electrical energy. While they can be built to almost any value of total charge capacity by increasing the size of the catholyte and anolyte reservoirs, one of their limitations is that their energy density, being in large part determined by the solubility of the metal ion redox couples in liquid solvents, is relatively low. Although an intriguing method of energy storage, such a system requires electrically conductive electrolytes which are often toxic or extremely acidic and corrosive.
Chiang, et al., U.S. Patent Application Publication 2010/0047671, which is incorporated by reference herein in its entirety, has recently disclosed an extension to the concept of flow batteries wherein slurries of electro-active materials distributed in ion-storing liquids which are themselves redox-active, are passed through so-called electro-active zones, providing one or two streams of chemically oxidized or reduced slurries. These separate slurries can be separated and stored until the need to recover the chemical energy, at which point the separate slurries are brought back together and the chemical energy is harvested, much like the process in the redox flow batteries. The stated advantage of this method is the ability to achieve much higher storage battery storage. The unstated disadvantage is that the method may suffer from many of the same problems of stationary batteries—i.e., discharging on storage, limited speed of electrical charge/discharge, and limited lifetime/cycle-ability. Other disadvantages include environmental hazards associated with redox reactive materials used in battery electrodes and electrolytes.
Thus, there remains a need for scalable high energy-density and high power-density energy storage devices.